This invention relates to apparatus and methods for exposing samples to reactive plasmas. More particularly, the invention relates to apparatus and methods for igniting plasma within a process module that is, for example, suitable for use in Chemical Vapor Deposition (CVD) and in Plasma Enhanced Chemical Vapor Deposition (PECVD) processes.
As used herein, a "sample" generically describes a substrate, such as a glass panel or a silicon water, which is suitable for deposition techniques, for example by PECVD. The sample typically has one surface that is treated to receive one or more film depositions.
Vacuum deposition systems which deposit semiconducting or insulating films onto samples are well-known and are utilized in a wide range of scientific fields. Complex PECVD systems have been developed, for example, to manufacture devices such as thin film transistors (TFTs), liquid crystal displays (LCDs), flat panel displays (FPDs), solar cells, photodetectors, and integrated circuit structures.
Typically, these systems include one or more evacuated process modules that are used to expose the sample to reactive plasmas. Such a process module typically includes first and second electrodes within a chamber and spaced apart to form a gap therebetween: one electrode is electrically grounded to the chamber, and the other electrode is connected to a radiofrequency (RF) source which generates a radiofrequency discharge within the gap. The module also has gas inlets and gas outlets which direct selected gases, such as silane, into and through the gap.
To deposit a selected film on a sample, the sample is suspended in the gap spaced away from the active RF electrode, e.g., mounted to the grounded electrode, and a selected gas introduced into the inter-electrode gap. When the gas is exposed to the RF field, it ionizes and forms a reactive plasma which deposits a film onto surfaces exposed to the plasma, including the sample surface. The rate at which this film is deposited onto the sample surface is dependent upon several factors, including: the magnitude of the vacuum in the process module; the electrode spacing; the power and the frequency of the RF energy; and the gas flow rate.
For practical reasons, many prior vacuum deposition systems utilize excitation frequencies of approximately 13.56 MHz. However, it is known to be beneficial to deposit films at higher frequencies, since faster deposition rates are realized. U.S. Pat. No. 4,933,203, for example, discloses the deposition of hydrogenated amorphous silicon at frequencies from 30 MHz to 150 MHz, realizing a 500% to 1000% increase in deposition rates over those attained with 13.56 MHz systems. Such increases correlate to improved manufacturing throughput and efficiencies.
Nevertheless, further increases in deposition rates are sought to attain further improvements in manufacturing throughput and efficiency. One known technique for increasing deposition rates is to decrease the spacing between the two chamber electrodes. However, the reduction of the spacing is not without limit. U.S. Pat. No. 4,933,203 discloses, for example, that the spacing has a practical lower limit of ten millimeters because of certain phenomena. These phenomena include difficulties associated with igniting plasma at small electrode spacings.
Accordingly, it is one object of this invention to provide apparatus and methods for depositing films onto samples from a vapor.
It is another object of the invention to provide improved CVD and PECVD apparatus and methods for igniting a plasma within process modules that have small electrode spacings.
More particularly, an object of the invention is to provide PECVD apparatus and methods which increase the deposition rate of films deposited on samples exposed to reactive plasmas.
Another object of the invention is to provide vacuum deposition systems that expose samples to reactive plasmas, and which attain relatively high production yield.
Further object of the invention is to provide such apparatus and methods for exposing samples to reactive plasma and which are relatively low in cost and reliable in operation.
Yet another object of the invention is to provide apparatus and methods of the above character and which also can etch sample surfaces.
These and other objects of the invention will be apparent in the description which follows.